The present invention is directed to control devices for bicycle transmissions and, more particularly, to a bicycle rear derailleur with a roller for guiding a control wire.
Rear derailleurs are commonly used on bicycles to move a chain from one rear wheel sprocket to another rear wheel sprocket so as to change the speed stage of the bicycle transmission. A typical rear derailleur includes a base member that attaches to the bicycle frame and a movable member supporting a chain guide, wherein the movable member is coupled to the base member through a link mechanism. The movable member moves relative to the base member so that the chain guide guides the chain from one sprocket to another. A control cable of the type having an inner wire that slides within an outer casing often is used to control the derailleur. Usually, the outer casing is fastened to the base member and the inner wire is fastened to the link mechanism so that pulling and releasing the inner wire (using a shift control device mounted to the handlebar) applies a force to the link mechanism. This, in turn, causes the movable member and chain guide to move back and forth to align the chain with a selected sprocket.
The control cable usually is mounted to the rear of the derailleur such that the inner wire extends forwardly to the link mechanism. As a result, the control cable must bend significantly as it travels from the handlebar mounted shift control device to the derailleur. This creates significant friction between the inner wire and outer casing of the control cable which, in turn, adversely affects the operation of the derailleur. Some derailleurs include a cable guide pin or roller as shown in U.S. Pat. Nos. 3,979,962; 4,306,871 and 4,470,823 to change the direction of the inner wire at the derailleur. While these solutions work well when the derailleur base member is rigidly fixed to the bicycle frame, they do not work as well when the base member is pivotably connected to the bicycle frame. For example, such pivoting rear derailleurs often are used in conjunction with a front derailleur that switches the chain between multiple front sprockets attached to the pedals. When the front derailleur switches the chain from one front sprocket to another, the resulting variation in tension on the chain causes the base member of the rear derailleur to pivot. Since the pivot axis of the base member is usually spaced apart from the cable guide pin or roller, the pivoting base member causes the tension on the inner wire of the control cable to change. This, in turn, causes the movable member to move laterally inwardly or outwardly relative to the base member, thus adversely affecting the proper alignment of the chain guide with the selected rear sprocket.